Three-dimensional objects generated by an additive manufacturing process are formed in a layer-by-layer manner. In one example of additive manufacturing, an object is generated by coalescing and solidifying portions of layers of build material. In examples, the build material may be in the form of a powder, fluid or sheet material. The intended coalescence, solidification and/or physical properties may be achieved by printing an agent onto a layer of the build material. Energy may be applied to the layer and the build material which is coated with the agent coalesces and solidifies upon cooling. In other examples, three-dimensional objects may be generated by using extruded plastics or sprayed materials as build materials, which solidify to form an object.
Some printing processes that generate three-dimensional objects use control data generated from a model of a three-dimensional object. This control data may, for example, specify the locations at which to apply an agent to the build material, or where a build material itself may be placed. The agents/build material may be intended to provide a particular appearance to that part of the object, for example a color, gloss or surface texture.